Enable universal accelerated overflow scroll

Source/core/rendering/compositing/CompositingTriggers.h

 /*
  * Copyright (C) 2006, 2007, 2008, 2009, 2010, 2011, 2012 Apple, Inc. All rights reserved.
  * Copyright (C) 2010 Nokia Corporation and/or its subsidiary(-ies).
  * Copyright (C) 2012 Samsung Electronics. All rights reserved.
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Library General Public
  * License as published by the Free Software Foundation; either
  * version 2 of the License, or (at your option) any later version.
  *
@@ skipping 15 matching lines @@
 
 enum CompositingTrigger {
     ThreeDTransformTrigger = 1 << 0,
     VideoTrigger = 1 << 1,
     PluginTrigger = 1 << 2,
     CanvasTrigger = 1 << 3,
     AnimationTrigger = 1 << 4,
     FilterTrigger = 1 << 5,
     ScrollableInnerFrameTrigger = 1 << 6,
     GPURasterizationTrigger = 1 << 7,
+
     // FIXME: This is a temporary trigger for enabling the old, opt-in path for
     // accelerated overflow scroll. It should be removed once the "universal"
     // path is ready (crbug.com/254111).
+    // Currently there is no way to enable this trigger, which means we can
+    // remove it once we're confident in the current codepaths.
     LegacyOverflowScrollTrigger = 1 << 8,
+
     OverflowScrollTrigger = 1 << 9,
     ViewportConstrainedPositionedTrigger = 1 << 10,
     AllCompositingTriggers = 0xFFFFFFFF,
 };
 
 typedef unsigned CompositingTriggerFlags;
 
 }
 
 #endif